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Burial, compaction, and porosities in a subduction zone
As part of the rock cycle, sediments become rock through the processes of
mechanical compaction and cementation, where precipitated minerals fill up the pore
spaces between sediment grains. These processes occur as sediments are buried beneath
other sediments. An environment where burial is extremely rapid is a subduction zone,
where sediments on one tectonic plate are carried beneath another tectonic plate.
In this assignment, you will look at information from the Nankai subduction zone,
which is located off the coast of southern Japan. Because of the subduction zone,
earthquakes are an important geologic hazard in Japan. To better understand subduction
zones and earthquakes, the Ocean Drilling Program (ODP) has drilled into these
sediments as they are being subducted. To see a site map, click on:
http://www-odp.tamu.edu/publications/190_IR/chap_01/c1_f1.htm
1) What plate is being subducted, and which plate is it being subducted beneath?
2) How quickly are the two plates coming together, relative to one another?
You are going to examine some data from two Ocean Drilling Program sites. The
locations of those sites (and some other nearby sites) are shown on the map at:
http://www-odp.tamu.edu/publications/190_IR/chap_01/c1_f6.htm
Site 1173 (on the right hand side of the map linked above (Figure F6)) drilled into
the sediments before they are subducted. Sites 1174 and 808 drilled through the
overriding plate and into the sediment on the incoming plate after they are subducted.
You may get a better feel for this by looking at the seismic reflection cross-section
through those sites, shown at:
http://www-odp.tamu.edu/publications/190_IR/chap_01/c1_f9.htm
In this area, the upper part of the sediment riding in on the subducting plate are
being scraped off and piled up on the overriding plate to form an accretionary prism. The
scraped off (“accreted”) sediments are faulted and folded. The lower part of the sediment
is being carried down, or subducted, with the ocean plate. These sediments remain
relatively undisturbed, and you can still see the horizontal bedding planes. The separation
between the two plates is marked by the “decollement”, which means “decoupled”. The
trough or “trench” that forms on the ocean bottom during subduction has been partially
filled with flat lying sediments that have been eroded from Japan.
Using the seismic reflection cross-section, answer the following questions:
3) Using the scale bar in the lower left hand corner of the cross-section, what is
the distance from Site 1173 from Site 808?
4) Using your answer to Question 2, calculate how long it would have taken
sediments and crust that used to be at the location of Site 1173 to be carried to
the locations of Site 808.
Next, you will look at what happens to the sediments that are subducted. These
sediments consist primarily of clay and silt that has slowly settled to the ocean floor. At
Site 1173, the sediments that will be subducted are at a depth of 420 meters below the
seafloor (mbsf). The estimated age of the sediments at 420 mbsf is 6 million years.
5) At Site 1173, what is the burial rate of burial for the sediments at 420 mbsf (in
meters per year)?
At Site 808, these subducted sediments are buried to a depth of 950 mbsf.
6) Using your answer to Question 4, calculate the rate of burial for sediments
being subducted as they move from the location of Site 1173 to Site 808 (in
meters per year). How does this rate compare to the burial rate at Site 1173?
Porosity is the percentage of open spaces in a volume of sediment. For sediments
deposited at the bottom of the ocean, these “open” spaces contain sea water. As
sediments are buried, the weight of the overlying sediment compresses the sediments,
squeezes out the water, and reduces the volume of open space. As a result, porosity
decreases.
7) For water to be squeezed out of the sediment, it has to flow through the
overlying sediments to the seafloor. What do you think would happen to
porosity if the sediment was buried faster than the water could escape?
8) What do you think happens to the pressure of the water if it is buried, but
can’t flow out fast enough?
Most sediments likely are between the freely drained (plate with lots of holes) and
completely undrained (plate with no holes) scenarios.
Water pressure is very important in subduction zones, because it can affect how
faults move, including the faults that cause earthquakes.
Porosities for Site 1173 sediments that will eventually be subducted are shown by
the “x” symbols in the figure below.
You can see that the porosity gets smaller with depth. The solid curve was fit to
the Site 1173 porosity data. Porosities for subducted sediments at Site 808 are shown by
the “+” symbols. The sediments from both Site 1173 and Site 808 shown above
accumulated in similar settings (all are mostly silt and clay). If they were freely drained,
the Site 808 sediments would likely follow the trend for the 1173 sediments.
9) What would the porosity of sediment at 1000 mbsf be if it followed the curve
based on Site 1173 porosities?
10) What is the porosity of Site 808 sediment at 1000 mbsf?
11) Explain why the porosity at Site 808 might be higher than expected from the
trend at Site 1173.
12) What would you predict about water pressures at Site 808? How could you
test that prediction?